1. Field of the Invention
The present invention relates to an image forming apparatus of, e.g., an electrophotographic method and an image processing method thereof. More specifically, the present invention relates to an image forming apparatus and image processing method for reducing the amount of printing material without deteriorating the image.
2. Description of the Related Art
In an electrophotographic printer (including an apparatus with a printer unit, such as a copying machine), when the amount of toner as a printing material exceeds a predetermined value, defective toner fixing or toner scattering sometimes occurs. Defective fixing and toner scattering not only deteriorate image quality but also damage the printer apparatus body. To solve this problem, toner reduction is performed in image processing before forming an image. Toner reduction is also referred to as TOR. As one of the toner reduction methods, a method has been proposed in which a color conversion processing unit manipulates values of a conversion table upon converting a colorimetric system from L*a*b* as a standard colorimetric system into CMYK as a colorimetric system of an output device (see, Japanese Patent Laid-Open No. 9-247471). In Japanese Patent Laid-Open No. 9-247471, a color conversion table in which CMYK color values corresponding to L*a*b* lattice points are registered is used to convert L*a*b* into CMYK. In this color conversion table, the CMYK color values are registered such that the total amount of the output colors of the lattice points becomes smaller than a limit value (e.g., 250% when the maximum value is 100%) permitted for output. When an image is formed, a printing material is used in an amount corresponding to a pixel value. Hence, the above processing prevents the total value of CMYK after color conversion from exceeding the limit value, thereby reducing the amount of ink or toner.
However, in a conventional image forming apparatus, the toner amount has been further reduced in a process between toner reduction and actual printing. This is because, in the electrophotographic method, an input pixel value (e.g., density value) and density of an image formed based on the pixel value do not have a linear relationship. In order to correct the nonlinear relationship of the input pixel value and image density to the linear relationship, the input pixel value is converted. The toner amount is further reduced due to this conversion. This conversion will be referred to as print tone correction, hereinafter. Print tone correction includes gamma correction and correction of a change with time of output density of a printer. In the electrophotographic method, the density of a formed image tends to become higher than the desired density as time elapses. To solve this problem, print tone correction is performed to nonlinearly convert the input pixel value such that densities, particularly intermediate densities, become lower. That is, when input pixel values are plotted along the abscissas and output pixel values are plotted along the ordinates, the conversion characteristic curve of print tone correction is concave downward; the print tone correction decreases the input pixel values.
When toner reduction is performed before print tone correction, a pixel value decreased by the toner reduction is further decreased by the subsequent print tone correction. FIGS. 4 and 5 show examples of the toner amount when a single-colored input image is printed in which the sum of the pixel values of each color component is 300% of the maximum value of each color component. FIG. 4 is a view showing the sum of the CMYK toner amount when no toner reduction is performed. FIG. 5 is a view showing the sum of the CMYK toner amounts when toner reduction is performed.
As shown in FIG. 4, even when the sum value of each color component (CMYK) of the input pixel value is 300% of the maximum value of each color component, it becomes 200% due to the print tone correction. When a limit value is 250%, the density is converted into that equal to or smaller than the limit value, without performing toner reduction.
On the other hand, as shown in FIG. 5, when the sum value of each color component of the input pixel value is 300% of the maximum value of each color component, since it exceeds a limit value of 250%, toner reduction is performed. As a result, the sum of the pixel values of each color component is converted into a value equal to or smaller than the limit value, i.e., 250%. After that, the sum of the pixel values is further decreased down to 180% by the subsequent print tone correction. That is, toner reduction is excessively performed. Excessive toner reduction results in deterioration of tone characteristics of image data and quality of a formed image.
In the above-described toner reduction technique, even when toner reduction is performed while setting a temporary limit value larger than the proper limit value as a threshold in consideration of the final reduction of the toner amount, it is difficult to reduce the toner amount strictly within the limit value.